Babies who are not breastfeeding usually drink from a bottle provided with a teat.
The bottle is usually hard. That is to say, it does not deform during feeding, nor can it be pressed together by the usual forces applied by hand. During feeding, therefore, air has to be able to enter the bottle. This is usually achieved by the threaded ring with which the teat is secured on the bottle not being completely sealed with respect to the neck of the bottle. If the threaded ring is twisted too tightly on the neck of the bottle, too little air can flow into the bottle, and the baby has to make too much effort when feeding. However, if the threaded ring is applied too loosely to the neck of the bottle, milk drips out of the bottle from the threaded ring.
DE 23 41 762 proposes providing the suction opening of the teat with a non-return valve and providing the securing flange of the teat with an air valve. The air valve is formed by a two-stage hole and by a U-shaped or V-shaped diaphragm section with an X-shaped or Y-shaped cut.
WO 2007/137440 discloses a teat unit with a teat and a dimensionally stable receiving head for receiving the teat. The one-piece receiving head is provided with a threaded ring, such that it can be screwed onto the neck of a feeding bottle or a drinking cup. The teat is mounted on the hemispherical receiving head and not secured to the threaded ring. The teat has an air sealing lip that can open and close an air opening in the receiving head.
US 2008/0237176, U.S. Pat. No. 2,529,794, U.S. Pat. No. 2,516,084, U.S. Pat. No. 2,084,099, U.S. Pat. No. 4,730,744, U.S. Pat. No. 5,499,729 and U.S. Pat. No. 4,865,207 disclose feeding bottles with teats, which feeding bottles are provided with an air inlet device at their end directed away from the teat.
U.S. Pat. No. 2,745,568 discloses a teat unit with a non-return valve and with a second valve. Liquid that has collected in the teat can flow back through this second valve into the bottle.